(1) Field of the Invention
The present invention relates to an improvement of modular type hot isostatic pressing (hereinafter referred to as "HIP") method and apparatus provided with preheating or cooling auxiliary stations for sintering or densifying ceramic or metallic powder at high temperature and pressure in an inert gas atmosphere to obtain a molded product having a dense texture of a nearly true density.
(2) Description of the Prior Art
The HIP treatment has recently been specially noted in various fields as a superior method for compressing a work isotropically at a high temperature using an inert gas as a pressure medium to produce a dense sinter from ceramic powder, metallic powder, or a mixture thereof, or for removing residual cavities in cemented carbides by squeezing, or for diffusion-bonding of metallic materials.
According to the HIP method, there can be obtained various advantages such as, for example, high densification at low temperatures, obtaining of a dense and uniform texture having a density close to a theoretical value, improvement of mechanical and physical properties of powder, molding of powder unsuitable for molding, producing of large-sized products not restricted by the capacity of a press as in ordinary molding presses, molding of various composite materials such as metals and ceramics, and improvement of the material yield. By the HIP treatment, moreover, internal defects of an object can be removed, and the toughness and deflective strength can be enhanced, so methods which utilize this effect have been proposed other than the above-mentioned powder molding and sintering, such as improvement of the performance of sintered tool material, etc. and diffusion-bonding of the turbine blade and body by HIP to obtain extremely strong bonding.
Since such HIP treatment is performed in an atmosphere of high temperature and pressure, it is necessary to use an HIP furnace of a special structure, and a long period of time is required for executing the operation cycle comprising raising the temperature, raising the pressure, maintaining the elevated temperature and pressure, lowering the temperature and lowering the pressure. Therefore, shortening this cycle time and thereby improving the efficiency has been an important technical problem.
In a effort to solve the above-mentioned problem, various attempts have been made for improving the utilization efficiency per unit time of the HIP furnace by performing heating in a preheating furnace to raise temperature which requires a long period of time and performing in the HIP furnace only the raising of the pressure and/or raising the temperature to a slight extent. A typical example is the apparatus proposed in the specification of British Patent No. 1,291,459. However, this proposed apparatus is disadvantageous in that the equipment cost is increased because a preheating furnace is needed in addition to the ordinary HIP furnace although the shortening of the cycle time is attained, in that the heat loss caused by heat radiation from the work is very large because the conveyance of the work after preheating is performed in the air, and in that when the high-temperature work after preheating is charged into the HIP furnace, the lower inner wall surface of the furnace is overheated and the lower seal ring is easily damaged thereby, which is a serious problem.
In this type of apparatus for which safety is strictly required, the adoption of the above-mentioned apparatus is very problematic even if the shortening to the cycle time is attained.
As the material of heating element used in the heater, usually an electric heater, in the HIP furnace, there has been proposed Fe-Al-Cr, molybdenum of graphite. Among these materials, Fe-Al-Cr, which is resistant to oxidation at high temperatures, has been evaluated as the only material capable of being released to the air at a high temperature, but the temperature at which this material can be used stably is up to about 1,100.degree. C.
On the other hand, molybdenum- or graphite-based materials which are stably employable at above 1,100.degree. C. are severely oxidized at high temperatures, so cannot safely be exposed to the air unless the temperature range is below about 200.degree.-300.degree. C. Therefore, a long period of time is required for lowering the temperature to below 300.degree. C. although the lowering of pressure can be done in a relatively short time period after performing the HIP treatment at a temperature as high as one thousand and several hundred degrees centigrade in a high pressure inert gas atmosphere. Thus, the long period of time required from opening the HIP furnace until taking out of the work greatly impedes efficient utilization of the apparatus. As an example, according to a certain conventional typical pattern in the HIP treatment, the time required for each treating step is as follows:
______________________________________ Time required Step hr. min. ______________________________________ Loading of workpiece 0. 10 Vacuum suction, Gas replacing 1. 00 Raising temp., Raising pressure 3. 00 Maintaining elevated temp. and pressure 2. 00 Lowering temp. 8. 00 Recovery under reduced pressure 1. 00 Taking out of workpiece 0. 10 Total 15. 20 ______________________________________
By the foregoing preheating, the 3 hours' temperature and pressure raising time is shortened to about 1 hour and 40 minutes, corresponding to only an 8.7% reduction of the cycle time, that is, the time required for lowering temperature, which occupies the greater part of the cycle time, still remains as a serious efficiency impeding factor.
For shortening the time required for lowering temperature, it has been previously attempted to perform cooling by providing a coolant jacket around the outer periphery of the HIP furnace and utilizing, in lowering the temperature, convection of gas induced by the difference between the specific gravity (small) of the high temperature gas at the furnace central portion and the specific gravity (large) of the low temperature gas in contact with the furnace inner wall, as disclosed, for example, in the specification of U.S. Pat. No. 4,217,087 and Japanese Patent Publication No. 8689/1973. According to such method, however, the cooling capacity deteriorates to a large extent with a decrease of the temperature difference between the high temperature gas and the low temperature gas. Therefore, the temperature lowering rate becomes smaller as cooling advances, and as a result, it is impossible to expect a remarkable shortening of the time required until reaching the temperature at which the HIP furnace can be opened.
In such technical level, the applicant of the present invention has previously proposed (see Japanese Patent Laid Open Publication No. 71301/1983) and HIP system capable of shortening the cycle time without exerting a bad influence on its components and having a high safety, as well as a method capable of improving the working efficiency remarkably by using such system. This proposed HIP system, called a modular type HIP system, comprises an HIP furnace, a plurality of auxiliary stations, the HIP apparatus and the auxiliary stations being disposed side by side along and above a horizontally laid track, and a carriage for travelling on the track. The HIP apparatus consists mainly of a high pressure vessel and a treating chamber and is provided with means for supply and discharge of an atmospheric gas for applying HIP treatment to a workpiece loaded into the treating chamber and is also provided with means for adjusting pressure and temperature, the high pressure vessel comprising a pressure-resistant vertical cylinder having a closed top removably fitted in the bottom of the cylinder, the treating chamber being enclosed with an inverted cup-like heat insulating barrier which barrier is mounted on the upper surface of the barrier and is internally provided with a heater. Each of the auxiliary stations mainly comprises a dome-like vessel having a size which permits the treating chamber to be completely enclosed therein, also having a bottom opening which permits the above plug to be fitted therein, and further having a coolant jacket provided around the outer periphery thereof. Each auxiliary station is also provided with the heater enclosed therein together with the treating chamber, means for supply and discharge of an atmosphere gas for heating or cooling the workpiece and temperature adjusting means.
Thereafter, the applicant of the present invention has made various improvements on the above-proposed apparatus and filed the thus-improved apparatus (see Japanese Utility Model Laid Open Publication Nos. 157,300/1983 and 54098/1984). According to these devices, in the foregoing modular type HIP system, a single valve mechanism is provided in the upper or lower portion of a casing which houses the treating chamber. This valve mechanism is opened when the treating chamber is inserted into the HIP furnace or an auxiliary station, to thereby provide communication between the interior and exterior of the treating chamber, and it is closed when the treating chamber is taken out. According to this construction, it is possible to take out the workpiece which has been preheated in the auxiliary station, from the auxiliary station integrally with the treating chamber together with the inert atmospheric gas, convey and load the workpiece into the HIP furnace, then after HIP treatment and upon dropping of pressure, taken out the workpiece from the HIP furnace integrally with the treating camber immediately without waiting for such becoming cold, and cool it in an auxiliary station. Thus, a remarkable shortening of the cycle time in the HIP treatment and a great improvement of the working efficiency could be attained. In these devices, however, since the casing which hermetically encloses the treating chamber is taken out from the HIP furnace also under a state of high temperature, there arises the foregoing serious problem that the seal ring attached to the lower portion of the high pressure vessel is easily damaged when opening the HIP furnace in a still hot condition of its interior and taking out the treating chamber held at a high temperature.